1. Field of the Invention
The present invention relates to a focus detecting device arranged to detect focus by illuminating an object of focus detection with an auxiliary light of auxiliary light projecting means and on the basis of an output of light receiving means which receives reflection light from the object, when the object has a low luminance in detecting focus, and also to a camera having the focus detecting device.
2. Description of Related Art
The focus detecting device of a camera or a like photo-taking apparatus is arranged to detect focus by leading the light of a photo-taking object incident on an optical system to a light receiving element. The focus detecting device has been variously arranged to detect focus, including a phase-difference (detecting) type. In the case of the phase-difference type, the focus of a camera or the like is detected, for example, on the basis of a phase difference between two optical images which are obtained by splitting incident light of the object into two images.
The phase-difference type focus detecting device is generally arranged to use an integration-type light receiving element. In the focus detecting device of the phase-difference type, the light of the object incident on a photo-taking lens is led to a pair of integration-type light receiving elements arranged in a secondary image forming system. The pair of light receiving elements are linearly arranged to accumulate electric charge according to the amount of incident light and to send out predetermined outputs as signals according to the accumulated electric charge. An arithmetic circuit is arranged to receive the signal outputs and to detect a state of focus (an amount of defocus) by computing a phase difference between the signal outputs. The state of focus thus obtained is used for focus adjustment.
However, in a case where the photo-taking object is dark, the arithmetic circuit is unable to obtain the signals with a sufficient amount of amplitude as the light amount incident on the light receiving elements is small. Under such a condition, focus detection cannot be made by the focus detecting device. To solve the problem, a focus detecting device was disclosed in Japanese Laid-Open Patent Application No. Sho 59-195605. This focus detecting device is arranged to detect whether or not the object of photo-taking has a low luminance and to control the light emitting action of a light emitting device according to the result of detection.
With the focus detecting device arranged in this manner, the focus detecting device performs a focus detecting action on a focus detecting area in accordance with the known phase-difference method.
FIG. 13 shows the state of signals accumulated by the light receiving elements (sensor array) with the flash auxiliary light of a flash device or the like used as the auxiliary light.
Referring to FIG. 13, the amount of light emission increases and an accumulation voltage which is accumulated at the light receiving elements rises as long as the object is illuminated with the flash auxiliary light. A maximum image signal shown in FIG. 13 represents the value of output of a cell which gives the largest image signal output among other cells included in a sensor array which is composed of a plurality of cells. A minimum image signal shown in FIG. 13 represents the output value of a cell which gives the smallest image signal output among other cells included in the sensor array composed of the plurality of sensors. As shown in FIG. 13, both the maximum image signal and the minimum image signal rise every time the illuminating flash light is projected. In the case of FIG. 13, a difference between the maximum image signal and the minimum image signal comes to exceed a predetermined decision value POS when the flash light emission is made for the eighth time, thus indicating that the accumulation of electric charge has come to an end. The decision value POS is a value at which the output image signals are sufficiently obtained to obtain a correlation to be used for a focus detecting arithmetic operation, because the difference between the maximum image signal and the minimum image signal is large enough to give a sufficient image signal amplitude.
However, the amount of auxiliary light emission for illumination is limited. Hence, the conventional art requires further improvement with respect to the following points.
In taking pictures at night, the auxiliary light often fails to reach objects due to far distances, etc.
FIG. 14 shows, for example, a case where the object of photo-taking is located at such a far distance that the flash auxiliary light emitted from the camera hardly can reach the object. In this case, the luminance of the object is low despite the flash auxiliary light emission which is made eight times. Therefore, the value of a signal from the light receiving element indicating a difference between the maximum image signal and the minimum image signal hardly becomes larger. The amplitude of the output image signals is too small to be used for focus detection. Thus, the focus detecting arithmetic operation is hardly possible.
In this respect, a focus detecting arrangement has been developed to increase the amount of auxiliary light emission for illumination in cases where the auxiliary light emission amount is too small, as disclosed in Japanese Laid-Open Patent Application No. Hei 6-289281. However, since there is also a limit to the increase of the amount of light emission, the flash auxiliary light still fails to reach the object in some cases.
FIG. 15 shows another case where the photo-taking object is something like a wall having a uniform reflection factor. In such a case, the focus detecting arithmetic operation cannot be performed as it is impossible to obtain a difference signal indicative of a difference between the maximum image signal and the minimum image signal.
In these cases, the arrangement of the conventional art cited above meaninglessly illuminates the object with the auxiliary light despite the impossibility of focus detection and is thus hardly considered to be efficiently using energy. Besides, the number of cameras of the kind having a built-in flash device and using its flash light as the auxiliary light is increasing these days. Since the electric energy of a battery is rapidly consumed by the built-in flash device, the conventional arrangement of focus detecting devices must be improved also with respect to the service life of the battery.
One aspect of the invention lies in the provision of a focus detecting device having control means arranged to cause an auxiliary light device to illuminate an object of focus detection with auxiliary light, if the luminance of the object of focus detection is low at the time of focus detection, by performing control in the following manner. If the output of a light receiving device for receiving reflection light from the object of focus detection is found to be lower than a prescribed value, the control means stops focus detection being made by controlling the auxiliary light device. Therefore, waste of energy can be minimized by stopping useless illumination in a case where the reflection light cannot be obtained in sufficient amount by illuminating the object of focus detection with the auxiliary light of the auxiliary light device.
Another aspect of the invention lies in the provision of a focus detecting device for a camera, having control means arranged to cause an auxiliary light device to illuminate the object of photo-taking with auxiliary light, if the luminance of the object of photo-taking is low at the time of focus detection, by performing control in the following manner. If the output of a light receiving device for receiving reflection light from the object of photo-taking is found to be lower than a prescribed value, the control means stops focus detection being made by controlling the auxiliary light device. Therefore, waste of energy can be minimized by stopping useless illumination in a case where the reflection light cannot be obtained in sufficient amount by illuminating the object of photo-taking with the auxiliary light of the auxiliary light device.
The above and other objects and features of the invention will become apparent from the following detailed description of preferred embodiments thereof taken in connection with the accompanying drawings.